1. Field of the Invention
This invention relates to improvements in the preparation of aromatic urethanes. More particularly, it relates to an improved process for preparing an aromatic urethane (hereinafter referred to simply as "urethane") which comprises reacting an aromatic nitro compound, an organic compound containing at least one hydroxyl group (hereinafter referred to simple as "hydroxyl-containing compound"), and carbon monoxide at elevated temperature and pressure in the presence of a catalytic system composed of a catalyst consisting of a platinum metal, a platinum metal compound, and/or a platinum metal compound-containing compound and a promoter consisting of a Lewis acid and/or a Lewis acid-containing compound.
2. Description of the Prior Art
Urethanes have heretofore been produced mainly by reaction of isocyanates with alcohols. In recent years, the production of isocyanates has become difficult partly due to the lack and rising costs of starting materials therefor and partly due to the high toxicity of intermediate formed during the manufacture of isocyanates. Consequently, there have been developed and proposed many novel processes for the preparation of urethanes without using any isocyanate. However, these newly developed processes involve several serious problems and have not yet been put into practice on an industrial scale.
For example, U.S. Pat. No. 3,338,956 describes a process wherein urethanes are prepared from alcohols, carbon monoxide, and nitro compounds with the aid of a rhodium chlorocarbonyl catalyst. However, this process is not economically advantageous in the preparation of highly pure urethanes because the desired product is obtained only in low yield even if the urethanation reaction is carried out for a long period of time in the presence of a large amount of the catalyst.
There has been proposed another process in which urethanes are prepared by reacting organic hydroxyl-containing compounds, carbon monoxide, and nitro compounds in the presence of a metal of Group VIII of the Periodic Table combined with a promoter comprising a salt of a metal capable of existing in two or more valence states (German Pat. No. 1,543,051). However, this process is useless from an industrial point of view because the yield of urethane is low even if mononitro compounds are used as the main starting material, and the use of dinitro compounds results in still lower yields.
Moreover, a process is known which used a catalytic system composed of palladium and a Lewis acid (U.S. Pat. No. 3,531,512). In this process, urethanes can be obtained in fairly high yield of 80-90% even by using dinitro compounds as the main starting material. In order to attain such a high yield, however, it is necessary to carry out the urethanation reaction under such severe reaction conditions as an initial carbon monoxide pressure of 190-350 kg/cm.sup.2 and a temperature of 190.degree.-200.degree. C. In addition, the process has an industrially serious drawback in that the Lewis acid, e.g. ferric chloride, used as the promoter exerts a considerable corrosive action on metallic materials such as iron, stainless steel, and the like. Consequently, it is essential to use a reactor made of or lined with glass or tantalum in order to realize the process industrially. However, the use of a glass or tantalum reactor under the above-mentioned high temperature and pressure conditions poses further technical and economical problems.
Thus, all of these processes use a catalyst consisting of a platinum metal and/or a compound thereof and occasionally a promoter consisting of a Lewis acid and/or a Lewis acid-containing compound, but the above-mentioned difficulties including the low yield of the desired product, the use of severe reaction conditions, and the corrosion of the reactor by the catalyst have kept them from industrial practice.